Noise barriers of this kind are described in published European Application EP 0 407 852 A2 15 (U.S. Pat. No. 5,040,352). They are excellent at preventing loose fragments from being formed and falling out. Strips, ornaments or shapes can also be embedded in the panels which can be seen by birds and so protect them from flying into the panels accidentally.
Austrian patent specification 263 298 discloses a translucent structural panel in which a partly metal-coated fabric is embedded by casting. The fabric reinforces the structural panel, but cannot prevent individual fragments from forming in the event the panel breaks; owing to the high strength of the plastic used in the structural panel, the thread breaks at the same time as the panel. In addition, the fine lights in the fabric prevent the structural panel from being transparent, so that the panels are suitable only as lighting elements and not as window-panes. Also, metal-coated fabric can cause reflexes or reflections which detract from the appearance of the panels and also may appear to be transparent from one side without being so.
DE-U-84 35 283 discloses transparent structural panels comprising external cables which trap and secure any parts which are produced by a fracture. The fragments in question are relatively large and can break into other, free fragments which are still relatively large. Conventional strips or the like can be added to these known panels as protection for birds.
The "Schweizer Ingenieur und Architekt", No. 13, (1988), pages 379 and 380 discloses sound-insulating panels which disintegrate into loose fragments in the event of breakage. To protect birds, vertical strips, e.g. 5 mm wide, can be etched on the panels. This additional complexity is expensive, increases the risk of damaging the appearance of the panel and is unsightly.
Since it is expensive to embed bird-protecting elements in the plastic panels, which are cast in one piece or are laminated by being made up of individual cast plates, in practice bird-protecting symbols or elements are often adhered to the outside of the plastic panels. However, the externally-adhered bird-protecting elements are subject to weathering and interfere with cleaning the panels. They have the further disadvantage of spoiling the appearance of the panels, and the advantage of transparency is largely lost. If individual silhouettes of birds of prey are adhered to the panels, they do not disturb the environment, but a few silhouettes of birds of prey are more for appearance than a protection for birds.